1. Field of the Invention
This invention relates to electric chain saws. More particularly, this invention relates to a speed reduction device for coupling the electric motor of an electric chain saw with the drive sprocket for the cutter chain of the saw.
2. Description of the Prior Art
Electric chain saws adapted for heavy cutting requirements require a gear drive to reduce the speed of the cutting chain while maintaining a high engine speed and maximum horsepower. For instance, a typical electric motor operating at 18,000 RPM, if directly coupled to a drive sprocket having six teeth 0.375 pitch chain, will produce a chain linear speed of 6,848 feet per minute (78 miles per hour). This would produce excessive chain "slap" and wear on the guide bar, as well as increasing the magnitude of any "kickback" of the chain saw.
Various attempts have been made to provide speed reduction systems for chain saws.
Irgens, U.S. Pat. No. 3,669,162, provides a chain saw with a gear drive. In particular, Irgens provides a compact gear drive for a chain saw which provides adequate power for heavy cutting requirements and which does not appreciably increase the weight of the chain saw. The gear drive includes a combination gear and cutting chain sprocket in the form of a ring which has, along the outer periphery thereof, a plurality of circumferentially arranged recesses which receive and drive the drive tangs of the cutting chain. In addition, the internal surface of the sprocket ring is provided with internal teeth which mesh with the teeth of a pinion gear which is fixedly connected to the drum of a centrifugal clutch and is rotatably supported on the output shaft of the engine. To minimize the size and diameter of the sprocket, the recesses are inwardly open and the pinion gear on the output shaft comprises two sets of axially spaced teeth separated by an annular groove. The drive tangs project through the sprocket ring and are received in the annular groove. The combination gear and cutting chain sprocket is supported in eccentric relation to the axis of the output shaft by a bearing in the form of a hub which is located interiorly of the combination gear and sprocket and connected to the engine crankcase. The internal teeth of the combination gear and sprocket ride on the outer bearing surface of the hub. Of course, an opening is provided in the hub whereby the teeth of the pinion gear come into engagement with the gear teeth of the combination gear and sprocket.
While Irgens provides a light and compact gear drive, the utilization of an internally toothed gear renders the gear drive susceptible to clogging. In particular, saw dust produced during cutting operations will tend to build up in the spaces between the internal gear teeth and ultimately clog the mechanism, preventing engagement with the pinion gear. This will necessitate disassembly of the saw in order to unclog the gear mechanism.
Haupt et al, U.S. Pat. No. 3,857,179, attempts to overcome this problem by shielding the gear drive from saw dust infiltration. Haupt et al notes that electric chain saws, which are provided with gear reduction between the motor and the saw chain, have the gears completely enclosed within a sealed gear housing so that saw dust, dirt and other foreign material cannot reach the gears. Haupt et al further notes that this sealing of the gear housing increases the cost of the gear reduction and also requires a separate drive sprocket for the saw chain to be mounted outside the gear housing. In order to overcome these problems, Haupt et al provide a gear reduction system between the motor and the drive sprocket which is not enclosed within a sealed housing. Rather, the gears are designed to provide their own protection against the infiltration of foreign matter. The large, driven gear is generally cup-shaped and includes a cylindrical side wall and a flat end wall. Gear teeth are provided on the inside surface of the side wall. A drive gear is mounted on the drive shaft of an electric motor and engages the gear teeth of the driven gear. The electric motor is mounted on the housing of the chain saw, and the drive shaft of the motor extends through a portion of the housing which is positioned adjacent the open end of the driven gear. Thus, the open end of the driven gear is substantially closed by a wall of the saw housing and the drive gear is positioned within the driven gear. The saw housing may further include an arcuate wall which extends around the outer periphery of the driven gear to further enclose the gears. A sprocket for the saw chain is mounted on the outside of the end wall and is drivingly engaged with the chain.
While Haupt et al provide a chain saw without a sealed gear box, the lack of a seal may still permit foreign matter to infiltrate the gear mechanism. Moreover, the internal gear teeth of the driven gear are difficult to make and, consequently, add to the cost of manufacture.
Siman, U.S. Pat. No. 4,010,544, disclose a power transmission system for a chain saw wherein a belt drive is used to connect a pulley mounted on the crankshaft of an engine to a pulley mounted on a stub shaft which drives the drive sprocket engaging the saw chain. However, this particular power transmission is only used in order to isolate engine vibration from the chain saw housing. In other words, the Siman patent is directed to chain saws powered by single cylinder fluid pressure engines. Such engines have come into wide use because of their light weight, low cost, reliablity and versatility. However, such engines have one major disadvantage, namely vibration. The vibration of such single cylinder engines is caused by: rotating inertia forces of the crank and parts that revolve with it; reciprocating inertia forces of the piston and parts that reciprocate with it; inertia torque of the reciprocating parts; inertia torque due to the pendulum motion of the connecting rod; and torque variations produced by the varying gas pressure acting on the piston. The torque produced by the pendulum motion of the connecting rod is small and is commonly neglected. The torque produced by the varying gas pressure acting on the piston can be reduced by multiple cylinder arrangements but only at higher cost, greater complexity and additional weight. Siman seeks to overcome these problems by eliminating certain vibration forces and by mounting the engine in such a manner that the remaining vibration forces are not transmitted to the housing. In particular, the rotating inertia forces are eliminated by the use of counterweights while the reciprocating forces are left unbalanced. These reciprocating forces tend to produce reciprocatory vibration of the engine in a direction axial of the engine cylinder. The engine, however, is isolated from the housing by a spring-loaded linkage system which permits movement of the engine relative to the housing in the direction of the axis of the cylinder while restraining movement in the transverse dimension. Power transmission is accomplished by the aforementioned belt drive, one pulley mounted on the crankshaft of the engine, and the other mounted on a shaft rotatably supported on the housing. Although the linkage system permits movement of the engine in the direction of the engine cylinder axis, there is no change in the center-to-center distance of the pulleys and the belt drive is unaffected by the engine movement. Alternatively, two gears can be used to transmit power, since the center-to-center distance of the crankshaft and the other shaft is maintained constant by the linkage.
Thus, it is clear that Siman is limited to the particular case of overcoming vibratory motion in the single-cylinder engine of a chain saw.
Irgens, U.S. Pat. No. 3,372,718, and Scharpf, U.S. Pat. No. 3,530,909, disclose bevel gear arrangements for driving the drive sprocket of a saw chain which merely comprise a pair of externally toothed gears.
Bross, U.S. Pat. No. 4,198,752, and Kolorz, U.S. Pat. No. 3,849,883, disclose the direct mounting of the drive sprocket on the motor shaft of a gasoline-powered chain saw.